This invention relates to (i) a method of preparing soft, fatigue resistant, elastomeric articles for transmitting load between moving mechanical parts through incorporation of certain isobutylene polymers, and (ii) articles prepared thereby. This invention, in preferred embodiments, relates to (i) a method of preparing soft, heat and fatigue resistant, elastomeric articles for transmitting load between moving mechanical parts comprising such isobutylene polymers and other essential ingredients and (ii) articles prepared thereby.
Elastomeric articles of the type to which the invention applies have diverse applications. Vehicular applications include, for example, suspension components such as front or rear suspension bushings, engine mounts, etc. Elastomeric articles or parts in these applications receive and transmit loads between mechanical components in relative motion with one another. The elastomeric articles, accordingly, require an optimal fatigue resistance as well as other desired properties.
It would be desirable in certain circumstances that the elastomeric articles or parts also be soft. For example, it would be desirable to have a vehicular suspension bushing that had lower hardness as compared to traditional suspension bushings. The lower hardness could alter favorably vehicle ride characteristics in, for example, lighter, smaller vehicles.
Physical alteration of an elastomeric part of this type so as to make it softer may have concommittant drawbacks. For example, an increase in the size of the elastomeric parts (e.g., bushing) generally reduces stiffness because, for a given deflection, the part will be under a smaller strain; an increase in size, however, is inconsistent with an objective of lighter, smaller vehicles. Moreover, introduction of holes into the elastomeric article also reduces stiffness; the holes, however, may introduce stress concentrations in the article.
An alternative is to formulate a soft elastomeric article; even here, however, there is difficulty. For example, traditional fatigue tests apply constant load or constant strain to an elastomeric article test sample. In a comparison between test samples of unequal moduli under constant, repetitively applied loads, the softer test sample undergoes greater strain; it, therefore, receives higher energy input. On the other hand, a constant strain test is more severe on the harder sample because an equal amount of strain in the harder sample requires greater energy input.
Test conditions that approximate equal energy inputs to hard and soft samples better compare basic fatigue life of the samples. Under such conditions, it has been found that certain elastomeric articles formulated to be soft do not have comparable fatigue life to harder production counterparts.
An exception to usually diminished fatigue life of soft elastomeric articles of the above described type now has been discovered. Certain polymers of isobutylene have been found, at certain levels, not only to permit softer elastomeric articles or parts but, also, to give articles with desired fatigue life.
This invention may be practiced fully without any appreciation of the theoretical principles underlying such a discovery. Indeed, this invention should not be limited by any characterization of such principles. It is believed, however, that the isobutylene polymer crystallizes during strain of the article. The strain induced crystals prevent or reduce crack or other flaw propagation. Prevention or reduction of flaw propagation enhances fatigue life. At the same time, the relative inertness of the isobutylene polymer to crosslinking allows it to soften the elastomeric article.
It is an object of this invention to provide soft elastomeric articles for transmitting loads between parts suitable for fatigue producing applications.
An additional, but independent, aspect of this invention is provision of soft, fatigue and heat resistant elastomers. In smaller vehicles, for instance, elastomeric articles may be closer to engine or engine related components. Thus, the elastomeric part may be exposed to high temperatures for prolonged periods.
In accordance with this additional aspect of this invention, it has been discovered that certain isobutylene polymers may be formulated with natural or synthetic polyisoprene rubber and elastomeric polybutadiene as well as other ingredients to yield a soft elastomeric article that meets or exceeds many commercially available goods in certain fatigue and heat resistant properties.
Accordingly, it is an additional, independent object of this invention to provide soft, heat and fatigue resistant, elastomeric articles for transmitting loads between mechanical parts.
This invention accomplishes these and other objects as will be apparent to those in the art from the disclosure hereinafter set forth.